Preferably, the blades should be made from 1.25 mm mild steel.
If this is not available, 1.00 mm will also do but then the
maximum allowable head will be slightly lower, see par. 4.1.

Since the blades will be soldered with brass later on, all
paint or zinc (if it was galvanised) or `roller skin' (the black
layer that is present on new, warm-rolled steel) should be
removed. Once the blades are cut and bent to shape, it is much
more work to clean them, so better do it on the piece of steel
sheet the blades will be cut from later. Use a steel scraper and
coarse sand paper for the last finish. Sometimes `blank' steel
sheet is sold, with a smooth silvery color. In that case there is
no roller skin. Real roller skin is almost black with a slight
blueish hue.

Eventually, the blades are bent from rectangular pieces of 61
x 19.0 mm. The corners should be straight since that makes it
easier to make a straight runner later on. The blades can be cut
in several ways:

With a steel plate cutter, a scissor-like tool with one
blade fixed to a bench and another one operated by a long
handle.

If there is a well-equiped metal workshop around, it
might be possible to buy accurately cut pieces of 61.0 x
19.0 mm with neat edges and straight corners.

It is important that all pieces have the same size and shape
as this makes it easier to built them into a straight runner
later on. Also the edges should be smooth since irregularities
could act as the start of a crack later on. Therefor cut slightly
bigger pieces of say 61.5 x 19.5 mm and file them to shape. About
15 can be clamped carefully aligned in the vice and filed in one
go. When using a cold chisel to cut the pieces, the initial sizes
should be even a bit bigger, say 62 x 20 mm. The runner needs 27
blades, but better make some more so that a few off-types can be
discarded.

For making a runner
with a high efficiency, it is recommendable to have the
inward side of the blades rounded. Then the water will
flow more smoothly around them after crossing the empty
inner space inside the runner. Rounding off blades could
best be done before bending them. If blades are rounded
at the inside, they could also stick about 0.5 mm more
inwards toward the center of the runner and therefor
become a bit stronger. Still blades of 19 mm wide can be
used, but now less will be filed off at the outer edge
when aligning the runner.

Remember to cut the slots in the side disks 0.5 mm
deeper when using rounded blades.

Fig. 4.4: Making the blades.

Fig. 4.5: Bending blades with a makeshift
press.

For bending them, you need a piece of shaft with a diameter of
28.2 mm (28.0 to 28.5 mm will do) and slightly longer than the
blades themselves, say 6580 mm. In a steel workshop, such a rod
can be made easily on a lathe. Then one only has to file a flat
side on it so that it holds better in the vice.

If you have to make one yourself, get a piece of 30 mm shaft
or a long 30 mm diameter bolt to start with. File 2 flat sides on
it until the thickness between the sides is just 28.2 mm. Then
start rounding off one of the curved ends until there is a smooth
curve with a constant radius from one end to the other.

Put the rod and a blade together in the vice (see fig. 4.4).
To prevent that the blade will bend in a screw-like shape, make
sure that the rod is well-aligned with the blade. Then slowly
hammer the blade around the rod. Turn the blade upside down to
hammer the other end into shape. It will look as if you can't get
the blades into shape because they will always bend back a little.
This is no problem since the radius of the rod was made slightly
smaller than the desired radius of the blades to compensate for
this effect. If you would hammer too long on the edges, the
blades won't be straight lengthwise anymore.

Blades could also be bent to
shape using a press as shown in fig. 4.5. It consists of
a 28.2 mm diameter rod and an outer mould that forms the
correct outer radius, made from a section of a suitable
size pipe. When operating the handle connected to the
rod, it will press downwards into the outer mould and
bend most of the blade. Repeating this after the blade
has been turned around will do the rest.

The side disks are very important parts. If they are built
correctly and accurately, it is relatively easy to build a strong
and straight runner that will work efficiently. Likewise, if the
shape of the side disks is lousy, it will be much more difficult
to build a proper runner.

The procedure described in detail below is meant for producing
a few side disks by cutting them by hand. To cut the strongly
curved slots for the blades, it makes use of jigsaw sawblades (the
ones designed for use in a jigsaw machine) that are fixed in a
homemade sawframe. Advantages are that little preparation work is
needed and one can see what one is doing, making it possible to
cut them reasonably accurately right from the start.

For making side disks in series, cutting by hand would take
too long and it becomes economic to cut them using a jigsaw
machine, see box 4.9.

Considering the time it
takes to cut side disks by hand, it could be advantageous
to buy a few side disks rather than making them oneself.
Since soldering in the blades is also a critical step, it
makes sense to buy a complete runner and maybe even the
nozzle. These items are light enough to send by air mail
and not too expensive. Prices excluding postage are: 2
side disks:  80, complete runner:  200,
nozzle:  130. See also par. 1.2.

The 2 prototype
firefly chargers had side disks without slots. The blades
were clamped in between the 2 side disks rather than
sticking through them. This construction saves the
trouble of cutting the slots in the side disks, but has
other disadvantages:

Soldering the blades becomes much more
difficult because one has to operate the flame
and brass rod in the narrow space between blades.
This gives a higher chance of poor quality welds.

Soldering will take longer, so more oxigen
and acethylene is needed.

Fitting 27 blades is virtually impossible
because then the space between the blades becomes
too narrow for soldering. So only some 20 blades
can be fitted, meaning that the maximum allowable
head will go down accordingly and turbine
efficiency will be a bit lower.

Without slots in the side disks, another way
of holding the blades in place during soldering
has to be made. For this, 2 jigs with slots for
the blades could be made. Those slots could have
a simpler form, the jigs could be made of thinner
material (so easier to cut) and they could be
used again and again. But still the time saved by
making side disks without slots would be limited.

Therefor this method is only adviseable for people
who have experience with brass soldering and have no time
to make them with slots. Then blades must be made 6 mm
shorter to arrive at the same runner width and only 20
blades can be fitted. Take into account that the maximum
allowable head will be only 11.6 m with 1.25 mm blades,
and 9.3 m with 1.00 mm blades.

The side disk drawn in fig. 4.6 shows the size and placing of
blades. The firefly runner deviates from crossflow design theory,
see box 4.7.

Fig. 4.6: Making side disks.

The side disk is drawn on scale 1 : 1 and indeed, it is quite
small. This leads to two problems:

To make a proper side disk, one has to work very
accurately. A vital step in this is to copy the design
onto the material from which the side disk is being cut.

The radius of curvature of the slots for the blades is
very small, too small to be cut with normal jigsaw
sawblades for metal (see the cross section of an original
sawblade in the lower drawings in fig. 4.7) or with any
other commonly available kind of sawblade for metal.

Answering the first problem comes down to not trying to
measure out all dimensions oneself, but merely copying it from
paper onto steel.

Make a copy of the side disk drawing of fig. 4.6. If the
copying machine is very old or seems unreliable, check
whether the dimensions are copied correctly: The outer
diameter should be 75 mm both in vertical and horizontal
direction. If it differs say 0.5 mm, this is still not a
problem as long as the horizontal and vertical
measurement are nearly equal.

Cut out the drawing and glue it onto the material. The
easiest way is by using double-sided Scotch tape but many
other types of tape or glue should do. Don't use glue on
water basis since it might make the paper expand
temporarily.

Mark all relevant points with a center punch through the
paper into the steel. Those points are the center of the
disk and all centers of curvature for the slots. Check
whether all center points are at the right position,
especially the one marking the center of the disk itself,
then remove the paper.

Mark all circles using compasses or a modified vernier
calliper (see box 4.8). These circles are:

The outer radius of the disk: 37.5 mm

The radius at which the slots end: 22.5 mm (or 22
mm if blades are rounded at the inside, see box 4.3)

The radius of the large central hole if it will
be the free side disk (see fig. 4.6): 10.0 mm.

The radius of the small hole if it will be the
alternator side disk: 3.0 mm. It seems
superfluous because this hole can be drilled
using the center point as a start. But because it
is important that this hole is well centered, it
is nice to have a mark once drilling has started
and the center point has gone.

The inward edge of all slots: 14.2 mm.

The outward edge of all slots: 15.6 mm. This
again might seem superfluous but it comes in
handy when the sawblade cuts into the slot: At
times the outward edge is better visible while
further on, the inward edge is easier to see.

To answer the second problem: Jigsaw sawblades have to be
grinded off until they can make such sharp curves. The easiest
way is by grinding off at the rear end only, see fig. 4.7, and
this is the best way if one wants to use sawblades in a saw frame.
It could best be done using a bench grinder. Standard sawblades
for metal could be used, but they have a rather short usefull
length of only 50 mm. It is worthwhile to look for extra long
sawblades that are more efficient for sawing by hand.

Without special measures, such a grinded off sawblade can not
be used in a jigsaw machine anymore because the rear end doesn't
touch the support wheel any more. In a jigsaw machine with an
orbital mechanism (meaning that the sawblade is pushed a bit
forwards on the upward stroke), the vertical, up and down-going
shaft might have so much play that the sawblade is still
supported by the support wheel. But then there is still another
problem: The rear end of the sawblade must have the right,
slightly conical shape, which is difficult to achieve once one
has been grinding off material at that end. So if one would like
to use a jigsaw machine, it is better to modify sawblades
according to fig. 4.10. See also box 4.9.

The shape of the sawframe itself is not so important, as long
as it holds the sawblade well, see fig. 4.7. When using extra
long sawblades, of course the sawframe should be adapted to this.

Fig. 4.7: Using modified jigsaw sawblades for cutting
slots in side disks.

Addition to internet version: Hacksaw sawblades can be used also.

In charger building workshops in the Philippines, people used hacksaw sawblades of
which about half of the height of the blade was grinded away and probably even more at the
side where it touches the outer edge of the slot. With such sawblades fitted in a normal
hacksaw, they could cut the slots quite fast. However, the side disks I have seen, had slots
with radiuses quite a bit larger than the design value of 14.2 mm and consequently, blades
will be less strong. Also the width at the teeth is only 1.0 mm instead of 1.4 mm for
jigsaw sawblades. One could try to widen the slot by cutting away material from one of the
sides, but this means extra work and a less smooth result. Normally, only 1.0 mm thick
blades will fit in (instead of 1.25 mm) and this makes the blades even weaker. If you are
interested, see the picture "Joel Cubit cutting a side disk" in:
http://www.microhydropower.net/mhp_group/portegijs/firefly_exp/ANEX_s.html

Some practical advice on cutting slots with a makeshift
sawframe:

Buy extra sawblades, to allow for blades breaking and to
be able to get a new one once it gets blunt. Count on
using one sawblade for each side disk that is cut and
keeping 1 or two spares.

After grinding off a sawblade, try it out on a piece of
scrap metal of 3 mm thick to see whether it can cut such
sharp bends.

Scrape off all paint, roller skin, zinc before starting
to cut. They should be clean before soldering and once
they are cut, it is more work.

Make sure to start cutting the slot in the right
direction. If one would `cut a corner' in the beginnen,
one can cut a more or less straight slot at the
beginning, but then has to cut an even sharper bend later
on and this is very difficult.

With a drop of oil on the sawblade, cutting goes easier.

The sawblades tend to get stuck because of the `waves' of
the wave-set teeth. Probably cutting would be easier with
a heavier sawframe, with the extra mass helping to push
it through the points where it gets stuck. To achieve
this, the handles of the sawframe could be replaced by
lengths of 30 or 35 mm diameter shaft.

Work at ease. When rushing things, quite likely the slots
will end up a lot less accurate. Anyway, cutting 27 slots
is quite a tiring job. With a sharp sawblade, cutting one
slot should take no more than 2 or 3 minutes, even when
working at ease.

Fix the side disk properly in a vice or by using clamps.

Sawblades might break, so wear safety glasses!.

Then the 6 mm hole in the center of the alternator side disk
can be drilled in (try to get it as well-centered as possible).
The 20 mm hole in het free side disk can be made by drilling
first with the largest drill bit available and then filing out
the rest (mark the 20 mm diameter circle before drilling away the
centre point that represents its centre). Or it can be made by
drilling a line of small holes along the edge, cutting out the
center bit and then filing it to shape.

*) These values can be
calculated from the other parameters
**) According to crossflow theory, this value
should be calculated from the blade angle at
outer circumference. Then for an angle of 36°,
it prescribes that inner radius / outer radius
should be 0.772

The most important difference of the firefly
runner with crossflow theory is its large radius of
curvature of the blade. This makes that the firefly blade
is stronger. But the main reason for it is a practical
one: Cutting slots with an even smaller radius of
curvature would become very difficult.

Having such a large radius of blades inevitably
means that inner radius ends up quite small. According to
crossflow theory, this is incompatible with the large
blade angle at the outer circumference since it will
create back pressure: At the inner radius, the space
between two blades becomes too narrow so the flow is
partially blocked there. So a pressure is needed to push
the water through this narrow area and then this pressure
can also be found at the outer radius. It is this
pressure that makes some water leak through the gap
between runner and nozzle.

I think this is not enough reason to do away with
the firefly runner design. In practice, a crossflow
runner designed according to the theory will also have
back pressure because:

A part of the flow is blocked by the blades
themselves (the theory assumes that blades are
infinitely thin)

The theoretical paths of water that enters
the runner at different points at the outer
radius, cross one another in the inner cavity of
the runner. Of course this cannot be: The
streamlines of water coming from different
directions will push one another away and again,
back pressure will result.

Tests showed that a firefly with the nozzle
designed for an entrance angle of 20°, consumed a flow
that was ca. 12 % lower than calculated. This means that
back pressure in the firefly must have been quite large.
Therefor nozzle design has been adapted so that it will
feed the same amount of water into the runner, but spread
it over a wider admission angle. So the new nozzle design
has an admission angle of 75° (was 60°) and feeds water
at an entrance angle of 16° (was 20°).

Now the entrance angle as produced by the nozzle
(= 16°) does not fit any more with the entrance angle
the runner was designed for (= 20°). In fact, the
difference is a bit smaller: Theory prescribes the form
of the `skeleton line' of the blade (= line right in the
middle between its inward and outward surface), while in
fact it is the inward surface that deflects the water. I
don't think this is a problem, I expect it will make that
blades will `scoop up' the water from the nozzle more
easily. Theory predicts that turbine efficiency would
rise by 4.6 % when runner design would also be adapted to
an entrance angle of 16°. But if one takes that theory
seriously, one should do away with the large blades of
the firefly and build the runner completely as prescribed
by the theory. No efficiency figures are available for
the turbine part alone, but an overall efficiency of up
to 0.388 for the complete charger suggests that not much
can be wrong with turbine efficiency (according to
HARVEY, 1993, alternator efficiency should be around 0.60
so turbine efficiency must be 0.65).

However, this issue deserves attention and maybe
in a future version, runner design will be adapted as
well. Reducing the entrance angle to 16° without
changing the inner radius would make the blades even
stronger. For people who want to build a runner this way:
Have the centers of curvature at a radius of 26.4 mm (instead
of 27.0 mm) and make the radius of curvature of blades 13.8
mm (instead of 14.2 mm).

The points at the jaws for `inside' measurements of a vernier
calliper can be used already as compasses for steel: Have
one jaw in a center point and scratch with the other over
the material. The material of a proper vernier calliper
is hard enough to scratch into steel without wearing out
too fast. Problem is that one doesn't know exactly what
radius the circle will get since the first jaw doesn't
align well with the center point. To solve this, the
vernier calliper can be modified a little: Adjust the
calliper to 1.0 mm exactly and fix it well. Now the
`inside' jaws largely overlap but the points themselves
are 1.0 mm apart. Grind off the overlapping inside jaws
until together they form one point shaped more or less
like the point of a center punch (say: with 90° top
angle). Now the vernier calliper can be used as compasses
and the radius can be set using its scale. The one thing
one has to remember, and is likely to forget, is that one
should add 1.0 mm to the desired radius because, when
adjusted to 1 mm, the points overlap again and the radius
is 0.

Because of its scale, such a modified vernier
calliper is easier to use and more accurate than real
compasses for steel. However, the points of its inside
jaws are modified and therefor `inside' measurements of
objects with only a small rim, will be more difficult.

For making series of
side disks, a sturdy jigsaw machine can be used with an
especially made foot plate that works as a circle guide.
The pin of the circle guide falls into the holes of a
mould that is clamped onto the side disk to be cut. To
make it possible to cut at some speed without overheating
sawblades, the sawblade should be cooled using drilling
fluid. Therefor a container with drilling fluid is
needed, with a support for the side disk being cut in the
middle. With every stroke, the sawblade dips into the
drilling fluid and is cooled in that way, see fig. 4.9.

Advantages of this method are that up to 2 side
disks per hour can be made, once everything works well.
Accuracy of the side disks can be very good and if there
are errors, quite likely all slots have the same error.
Disadvantages are the investment costs of the jigsaw
machine, it will take days of work before the first side
disk can be cut, probably the first ones aren't that good
and it takes quite some metal working skills to get
everything to work properly.

For use in a jigsaw machine, it is better to
modify the sawblade according to fig. 4.10. To make that
the support wheel will function well, the rear end of the
sawblade should as much as possible keep its proper shape.
So only at the outward, rear corner a bit is grinded off
on a bench grinder. Take care with grinding off the
inward side because the teeth themselves should not be
touched. This can best be done with a small angle grinder.
Clamp the sawblade horizontally in a vice, with a piece
of steel sheet between its teeth and the very hard jaws
of the vice. Fasten the vice only lightly because
otherwise the teeth will break. This piece of steel
sticks out above the sawblade and because the grinder
disk has a rounded edge, it won't reach the teeth
themselves easily.

In Holland, special narrow sawblades with 3 mm
pitch are available and it is better to start with these
rather than the standard metal sawblades with 1.2 mm
pitch. Because they are less wide, these blades can make
sharper bends from themselves and only a little has to be
grinded off at the inward side to make them suitable for
cutting slots. The 3 mm pitch makes them more suitable
for cutting through thick steel.

There is another way to get large numbers of very
accurately made side disks: Having them made by a
specialised workshop that has laser cutter equipment.
Such equipment will only be available in western
countries or the largest industrial centers of the third
world. With this method, having the side disk shape
programmed into the machine is costly, but once
everything is set up, prices could be quite low.

Fig. 4.9: A jigsaw machine adapted for
cutting side disks in series.

Fig. 4.10:A modified jigsaw sawblade for use
with a jigsaw machine.

When a good jigsaw machine is available anyway, it would be
tempting to use that instead of cutting by hand, but without
investing in making the foot plate with circle guide etc.
described in box: Making side disks in series. I think this is
not recommendable because:

Then sawblades have to be modified in a more complicated
way, see above.

Three mm steel is about the maximum a jigsaw machine can
handle. Quite some force is needed to push the machine
ahead. Problems could rise with respect to the jigsaw
machine vibrating too much, sawblades breaking and
sawblades becoming blunt prematurely due to overheating.
At least a jigsaw machine with speed control is needed
and one should use it at the lowest practicable speed (watch
out for overheating the machine) and keep the sawblade
oiled.

The blades can be soldered in the side disks with brass, a
yellow colored alloy of copper and zinc. Often this is called
`soldering with bronze', but in fact bronze is colored more
brownish (bronze) and is an alloy of copper with tin. Apart from
brass, there are several alloys of silver, with their proper
flux, that can be used for soldering. Probably they are even
stronger and easier to use, but more expensive than brass.

Quite likely, workshop people will suggest to use acethylene
welding (so: Heating until the steel itself melts and using iron
wire to add extra material) because it is cheaper. This is not
recommendable: It would result in a much weaker runner because it
is impossible to make the weld as smooth at the inside as can be
done with brass. Also the runner would become distorted because
of heat stresses.

For soldering with brass, an acethylene set is needed. It
consists of two steel bottles (with acethylene and oxigen
respectively), pressure reducers, hoses and the torch itself.
Acethylene sets are mostly used for cutting steel but with a
different torch, it can also be used for acethylene welding of
thin steel or, in this case: Hard soldering. Acethylene equipment
is quite expensive but essential for car and truck repair
workshops and many types of metal working workshops. If the
owners of such workshops are reluctant to cooperate, try a
technical school. Of course also the brass rods themselves are
needed, and a `flux' powder called `Borax' or `Boracit'.

It requires experience for working properly with acethylene
equipment, not only for adjusting and operating the torch and the
soldering itself, but also for the safety aspects. So for good
reasons, workshop people probably won't allow you to do the
soldering yourself. But insist that you are present during the
soldering and make sure that the soldering is done according to
the guidelines set out below.

The strength of the runner depends largely on the quality of
soldering, see box 4.10. Of course the skills
of the one doing the job are important, but things are a lot
easier if the material is clean and if there is soldering flux
everywhere where the brass should come to make a proper joint.
Using brass rods with a core of flux is not enough: Once the
joint is fully covered with brass from above, no more flux can
flow down to clean the area where it matters most. Therefor:

Before assembling the runner, make a stiff mash of flux
powder and water and rub some into the slots of the side
disks.

Once the runner is assembled, put it in a container, make
a more liquid mash and add as much that the lower side
disk is submerged a few mm. Before turning it around to
have the other one submerged as well, shake off the
excess flux (a droplet of flux mash flowing down a blade
could be followed by a droplet of brass during soldering,
which is very hard to file away and would partially block
the water flow later on). Then the runner can be dried.

This should make sure that there is ample flux everywhere.
This flux powder melts when everything is heated up and then acts
as a cleaning agent. It makes the brass creep into the smallest
crevices and form a smooth, rounded weld at the inside of the
runner where the blades stick out of the side disk.

Impurities like metal oxides will dissolve in the melted flux
but when there is too much impurities around, it might not be
able to deal with it all. The least that will happen is that the
flux will become black instead of transparent, making it
impossible to see the brass underneath it. That is why the roller
skin has to be scraped off from the material for both the blades
and the the side disk, even though the real joint will be in the
slots and there is no need to cover the surfaces of the side
disks with brass.

Before soldering, the runner must be assembled. Take the
direction of rotation of the alternator into account, check this
with fig. 4.6.

Usually either the blades are bent a bit irregular or the
slots turn out to be slightly off-shape and this makes that the
blades will fit quite tighly into their slots. They have to be
hammered inwards from the outside towards the center of the
runner. Use a light hammer and hold the runner in your hand while
hammering. If blades fit too tightly and the lips between slots
become bent while hammering in blades, try to bend the blades to
shape better.

Once all blades are hammered in, it is difficult to change its
form. So first fit a few blades at 3 places around the side disk
and check whether the distance between the side disks is correct:
55 mm. Check at several points around the circumference.
Hammering the side disks towards one another is easy, but watch
out that the lips between slots for blades are not bent. If the
side disks have to be pulled apart, stick a rod through the hole
in the free side disk and with that, hammer the alternator side
disk outwards. Also have a first check on whether the runner is
straight, see below.

When all blades are fitted, first check whether no blades
stick out through the side disks. If blades are too long or too
short, have them well aligned at the alternator side disk. This
surface should be flat since this is where the runner will be
clamped against the rim of the pulley. Also forces are greatest
at the alternator side disk, so if a blade is too short, it is
better to have the free side disk being weakened by a blade that
is a little sunken in.

Then check carefully whether the runner is straight: Hold a
carpenters square against the alternator side disk (the one with
the 6 mm hole in the center) and see whether the blades are well
aligned with it. Check at least 4 point around the circumference.

Now the runner might look crooked even though it is as well
aligned as possible if:

Not all slots are cut until exactly the same depth (this
determines the inner radius of the blades).

Some blades are slightly wedge-shaped.

If this might be the case, check a few blades at each point
and use the average. Or compare the circumferences of both side
disks rather than the blades. Of course then the outer radius of
both side disks should have been cut properly to give reliable
results.

To correct the alignment if a deviation was found, hold the
runner in your hand and hammer at the end of the side disk that
is sticking out. Normally, blades will stick out a little
radially so it will be a blade that has to absorb the blow. It
doesn't matter if the blade is deformed a little since this part
will be filed away later.

If the blades fit so tight that the runner can't be hammered
to shape in this way, try fixing the alternator side disk on a
flat, heavy piece of steel in the way it will be fixed later to
the alternator shaft (so with an M6 bolt and the 20 mm washer).
Then use a rod to hammer against the inside of the 20 mm hole in
the free side disk rather than hammering against the outer edge.

There is an easier way to get the runner well-aligned: Fit it
on the alternator shaft and check whether it wobbles. But then
first the alternator shaft should be made fit for this (see par.
4.4) and somehow it seems more logic to build the runner first.
If you want to use this method:

Fix the M6 bolt only very loosely, as without the blades
being soldered, the alternator side disk will bend easily.

Clamp the alternator onto a table and hold a carpenters
square upright from the table to see whether parts are
well-centered.

First get the alternator side disk well-centered, see
with aligning the runner in par. 4.4.

Now hold the square against the free side disk and get it
well-centered using light hammering. Heavy blows could
damage the alternator bearings and most likely are not
necessary anyway.

Don't be satisfied too soon in getting the runner well-aligned.
If it is not, then later a lot has to be filed off from the
blades at the end that is sticking out and those blades will
become significantly weaker.

During soldering, 2 side disks and 27 blades have to remain in
the right position with respect to one another. Usually the
blades fit quite tightly in the slots but to be sure, better put
a few windings about 1 mm iron wire around it. If still a blade
is a bit loose, make a mark with the center punch on the side
disk close to its slot. Then at that spot, the slot will become
narrower and the blade will stick well. Take care that the runner
will not become deformed again so pack it properly when
transporting it and when it is dropped accidentally, check the
alignment again.

Then about the soldering itself. Below, it is written as if
you will do the soldering yourself while in fact probably someone
of a workshop will do it. At least make all preparations you can
and try to explain how you would like to see it done.

Before soldering, submerge both side disks in a quite
liquid mash of flux as described above. Still it might
make sense to use brass rods with a core of flux, or to
add flux by dipping the hot rod into a container of flux.

To melt brass, the temperature should be well into red-hot.
I could see things better using the strong type of
sunglasses that are used in mountaineering.

Like with welding, the flame produces light by itself.
But it has a reddish hue and I produced much better
solderings if there was ample ambient light (full
sunlight or strong lamps).

To reduce heat losses, make a double-walled container of
1 liter tins, with at the bottom a small tin cut off at
such a height that the side disk to be soldered just
sticks out a little, see fig. 4.11. The double wall can
be made by cutting out both top and bottom of a tin and
then bending it until it fits in another tin of the same
size. Without such an isolation, one needs a larger
flame, will consume more acethylene and making good
solderings is more difficult because the risc of
overheating is much bigger. The double-walled container
will especially reduce heat losses from the blades. This
is important since the blades will tend to stay
relatively cool since they have a small surface area that
can be heated (the area that sticks through the side disk)
and a large area that radiates away heat.

Because of the isolation, the flame doesn't need to be
that large. Try the smallest torch head from the set (no.
0, with a hole of ca. 0.8 mm diameter) first and adjust
the flame so low that it is just able to heat up the
material so much that the brass will melt. In this way,
chances of overheating the brass (see below) are low, one
can work at ease and the use of acethylene gas is minimal.

Find a sheltered, quiet place to work. Put the container
at a stand at a convenient height and in such a way that
one can walk around it while soldering (otherwise one has
to turn the hot container for soldering from the other
end).

For having all blades soldered well, at least 15 g of
brass is needed for the two sides of one runner and more
if slots ended up wider than 1.4 mm or if 1 mm blades
were used. Normally quite a bit extra is needed because
the brass will not be divided evenly over all welds and
some will remain at the top surface of the side disk, so
plan on using 20 - 25 g per runner. Weigh or calculate
how much brass is in one brass rod (density of brass: 8.5
g/cm3) and plan how much of a rod you want to consume for
soldering one side disk.

Have pliers and a piece of iron wire with a hook ready to
lift the runner out of the container and turn it over.

Solder the free side disk first. This gives you an
opportunity to practice before soldering the alternator
side disk, which is more important from a strength point
of view. If you feel unsure, practice on pieces of scrap
metal first, having the same kind of joint to make (a
thin blade sticking through a slot in a thick piece of
steel) and using the container.

Adjust the acethylene/oxigen ratio of the flame until the
blueish cone that comes when there is an oversupply of
acethylene, has just disappeared. Check whether the flame
is formed regular and pointed. If not, clean out the hole
in the torch with the special `torch drills' that go with
it. - The flame should be adjusted so large that
reasonably fast soldering is possible while the risc of
overheating material is minimal. I had it so large that
when soldering the edge of the first blade (see `go 1'
below), it took about 5 seconds before the material was
heated up enough (with subsequent blades, it will take
less because they are pre-heated by the far end of the
flame). When letting the brass sink into the joints, the
flame was just large enough to keep the brass melted for
the whole length of the weld.

During soldering, use the white-hot cone of the flame to
heat up the material. For applying a droplet of brass,
also bring the end of the brass rod into this hot cone. -
A warning against overheating: The white-hot cone of the
flame is hot enough to melt steel and once it happens, it
is impossible to get it back into shape. A more important
effect is that overheating will make the zinc evaporate
from the melted brass, so that only red copper will
remain and this is much weaker than brass. There is only
a small margin between heating up the material enough to
make brass melt and heating it up too much to make the
zinc evaporate. Only when the runner has cooled down and
has been cleaned, you will find out whether it has been
too hot. So:

With ample ambient light, it is easier to see
what is going on.

Keep the flame constantly moving, even if you
want to heat up a small area. Generally for
soldering, the flame should never rest on one
place.

Fig. 4.11: Soldering a runner.

Then on the actual soldering itself. First one could heat up
the whole of the side disk for say 15 seconds, moving the flame
around from above in large circles. I got the best results by
working in 4 goes:

First make small joints at the outer circumference.
Direct the flame almost horizontally from the outside
towards the side disk and blade. In this way, one can
heat up both the side disk and the blade and apply a
droplet of brass once they are hot enough. Actually,
there are 3 parts to heat up: the blade and the parts of
the side disk at both ends of it (so there are 2 joints
to make between these parts). These 3 parts will conduct
away heat differently. So take care to direct the flame
in such a way that they heat up evenly. I kept the flame
more or less in line with the outer end of the blade (so:
Not towards the centre of the runner, but at an angle).
In this way, the next blade is already pre-heated by the
far end of the flame and one can work faster. Once there
are 2 small joints there, this will conduct heat from the
side disk (that is heated up easily by the flame) to the
blade (that will tend to remain too cool).

Then direct the flame from above to the side disk and
apply brass to the length of the weld. Do not bother
whether the brass sinks into the weld properly, but try
to apply the right amount of brass (see above) and to
apply it as regularly as possible.

Now heat up all welds until the brass sinks into the
joints properly. Apply some more brass when it looks like
there is only a very thin layer left on top. Move the
flame in circles over the weld you are working on, while
directing it already towards the next blade.

With the flame directed horizontally towards the edge,
check the welds at the outer edges of the blades again (this
is the critical point with regards to strength!!). Apply
extra brass when necessary. Ideally, at the bottom end of
the side disk, there should be a smooth, rounded weld
between blades and side disk, with a radius of say 0.5 to
1 mm. Mind that you can file away excess brass there
later, but can not add it.

When satisfied, turn it around and solder the other side disk.

When soldering is finished, take it out of the container and
let it cool down for a few minutes. Never pour water on very hot
objects as it might affect the quality of the steel. Only once it
has cooled down to less than say 200 °C, you might let it cool
down faster by pouring water over it.

The strength of the runner
determines the maximum head (without blocking timber) the
charger can stand without risking that blades will break
out. Don't be fooled by how sturdy the runner looks and
in fact is for single test forces. It is a fatigue
strength problem: With every revolution, a blade
experiences forces from the water when it rotates past
the nozzle, and no forces when it is somewhere else (the
forces when it is in the area where the water leaves the
runner, are smaller and unimportant in this respect).
Then after many hours of satisfactory functioning, the
first blade might come out. This would make that the next
blade is hit by twice as thick a slice of water from the
nozzle, so not long after, this blade will break and so
on. Only when a runner has survived 10 million
revolutions (83 operating hours at 2000 RPM) at a certain
head, it is safe to assume that this runner will last
forever at that head (or rather: Until the blades have
become noticeably thinner due to oxidation or the
grinding action of silt in the water).

The critical point in this is the joint of the
blades with the alternator side disk at the outer radius.
Here, stresses in the blade are highest so a crack is
most likely to develop here.

There is literature on this strength problem in
crossflow turbines (e.g. VERHAART, 1983 and VAN DER
VELDEN, 1985) and using Verhaart's method for calculating
the strength of a firefly runner, the maximum head would
be only 13 m (instead of the 15.6 m stated in par. 4.1).
However, as far as I can see, those calculations are not
directly applicable to the firefly design because:

These strength calculations are all based on
the assumption that the side disks to which
blades are fixed, are completely rigid. I don't
think this assumption is valid and probably
material stresses at the critical point would end
up a little lower if one would take this effect
into account. But for sure one would end up with
a much more complicated calculation that at least
I won't be able to work out.

The shape of the joint is very important for
the strength of it. Literally translated from
Dutch, `scratch-action' plays a role in this: A
carve or a sharp corner could act as the
beginning of a crack. The most favourable
condition is a smooth continuation from the blade
to the side disk, with its surface having a
radius of say 1 mm. In a perfectly soldered
firefly runner, this could be the case. The worst
situation is when the seam between blade and side
disk is not filled completely so there is a
crevice that could form the beginning of a crack.
This condition is almost inevitable for normal
crossflow runners with blades fixed by electric
arc welding from the outside. Assuming that those
calculations are checked with results obtained
with normal crossflow runners, one could expect a
perfectly soldered firefly runner to be
significantly stronger than an electric arc
welded crossflow runner. Comparing both the (perfectly
soldered) firefly runner and the electric arc
welded standard crossflow runner with fatigue
strength data from NEN 2063, 1988, one could
conclude that the firefly should be able to stand
roughly twice as high a head as these
calculations predict.

The firefly runner differs in another respect
from standard crossflow runners: It is mounted at
the end of a shaft so it is supported at one end
only. Standard crossflow runners are mounted on a
shaft sticking right through them with bearings
at both sides. This effect makes that the firefly
will be less strong than standard crossflow
runners, but how much less is difficult to say. I
didn't try to make calculations on this.

So using strength calculations designed for
standard crossflow runners and checked with experiments
on these, gives little certainty about the maximum head
for a firefly runner. Results with the first firefly
prototype gives some information:

It was used at a head roughly 10 % above what
was allowable according to VERHAART's calculation.

It was made hastily and roughly. No slots
were made in the side disk so soldering had to be
done between the blades. Blades were held in
position by someone else using pliers. So for
sure some blades sticked out too much and were
weakened significantly when the runner was filed
to shape.

It ran at a speed higher than the optimal
speed (then forces on the blades are less).

Only after at least 100 operating hours, it
turned out that a blade was missing. Eventually 3
blades had broken out by the time a new runner
was made for it. But contrary to what one would
expect, the second and third blade did not break
out soon after the first one.

This is why I think that a reasonably well
soldered firefly runner should be able to stand a maximum
head of 15.6 m, so 20 % higher than the maximum head
according to VERHAART's calculation.

To find out more precisely what is the maximum
head for a firefly runner, I want to test a perfectly
soldered runner at very high head until it breaks. I can
not afford to test it for many hours but there is no need
for this: If I know that it survived for so many
revolutions at a certain head, I can calculate back how
much lower the head should be for it to last those 10
million revolutions (meaning: forever).

This test should tell what the maximum head is for
a perfectly soldered firefly runner, let's call this `100
% strength'. Probably this 100 % will end up somewhere
between 20 and 30 m. Then one could rate runners
according to the accuracy of building and the quality of
soldering:

Perfect runner, 100 % strength: Blades and
side disks are made so accurately that deviations
from the design do not influence strength
noticeably. Critical dimensions like the radius
of curvature of the blade, the inner radius of
the blades and the outer radius of the runner
itself are accurate up to say 0.2 mm or deviate
towards the safe side. The inner side of the
blades are rounded (see box 4.3) and therefor
blades stick 0.5 mm more inwards than drawn in
fig. 4.6. Also with aligning the runner, the
proper shape of the blades was maintained. All
blades are soldered in perfectly, meaning that at
least at the critical point (outer radius at
alternator side disk), there is a smooth
continuation of the blades into the side disk
with the ideal radius of ca. 1 mm. At these
points, the shape of the joint is improved
further by filing in a little with a small round
file.

Good runner, 70 % strength: Made as
accurately as described above and with blades
rounded at the inside edge. At the critical
point, some of the joints do not have the smooth
continuation but a mere right angle between blade
and side disk. There are no partially filled
seams, leaving crevices that could form the
beginning of a crack.

Normal runner, 50 % strength: Made reasonably
accurately according to the design of fig. 4.6.
Seen from the top, all blades are soldered in
well, but at the critical points, not all blades
have the desired smooth continuation. From one or
a few blades, the seam between side disk and
blade might not be fully filled with brass,
leaving a crevice that could form the beginning
of a crack. However, then the next blade in the
row should have a properly soldered joint,
including the desired smooth, rounded brass weld.
On the surface of the side disk, there might be
spots where the brass has been overheated (showing
the red color of copper), but not on the critical
points.
The blade(s) with a crevice at the critical point
might break out but then the runner could still
be used without problems and the next blade can
stand the double forces.

Poor runner, 30 % strength: Made quite
inaccurately: With one look one can already see
which blades were placed wrong or have too little
curvature. Or with the aligning, a few mm of the
outer edge of blades at one end was filed off.
Quite a number of blades might have crevices at
the critical point or even from the top, it shows
that some blades are not soldered in well.

Rubbish: Anything less than a poor runner is
not usable.

This quality rating system implicates that a
runner with one or more blades missing is still usable,
as long as no two subsequent blades in a row are missing.
However, a runner with a blade missing can be a `normal'
runner at best (it is quite unlikely that one will come
across a `perfect' or `good' runner with just one blade
missing: When operating at a head that was high enough to
make one blade break out, the next blades in the row will
soon follow.

Please note that this quality rating system is
based on an `educated guess'. The `perfect runner' is
well defined and a maximum head test on another `perfect'
runner will give about the same head: Results are
replicable. Once inaccuracies and soldering errors come
into play, it becomes very hard to predict strength. One
runner rated as `poor' could in fact turn out to be
stronger than a `normal' one in which inaccuracies and
soldering errors cooperated to make two subsequent blades
rather weak, resulting in the third, fourth etc. ones to
break out soon after since these will get a progressively
thicker slice of water to deal with. This also means that
it makes little sense to test the maximum pressure of one
or 2 `normal' runners: Results could vary so much that
one should test 10 or more before one could draw
conclusions about the strength of these runners.

Normally, a safety factor is included in strength
analyses. Here I didn't because:

If a blade breaks out, it won't cause a
disaster. It is extremely unlikely that someone
will get hurt by a broken out blade. Also, the
machine is rugged enough to run with one blade
missing so it is still useable.

A safety factor is often used to allow for
damaging effects that have not been included in
the strength calculation, e.g. deviations from
the design, `metal fatigue' in parts that
experience repetitive forces, this `scratch
action' that is difficult to account for. So in
that case, for sure a part would not survive if
it was loaded so heavily that no safety margin
was left (meaning factor is 1.0). In this
discussion however, those effects have been
included as much as possible and the majority of
runners should survive when loaded so heavily
that safety factor is 1.0.

I would like to leave it to the user to judge
how well a runner was made and how heavily he/she
dares to load it.

So the maximum head of 15.6 m that was given in
par. 4.1 is rather arbitrary. Probably it leaves a safety
factor of about 2 when comparing it with the maximum head
of a perfect runner, but one can only be sure after such
a perfect runner has been pressure-tested. Then still the
safety factor for lower quality runners will be much less.
Probably only firefly runners that rate `good' or
`perfect' will stand the maximum head of 15.6 m
indefinitely.